Processes for the production of electro-optic displays, and color filters for use therein

ABSTRACT

Processes are provided for depositing multiple color filter materials on a substrate to form color filters. In a first process, the surface characteristic of a substrate is modified by radiation so that a flowable form of a first color filter material will be deposited on a first area, and converted to a non-flowable form. A second color filter material can then be deposited on a second area of the substrate. In a second process, first and second color filter materials are deposited on separate donor sheets and transferred by radiation to separate areas of the substrate. A third process uses flexographic printing to transfer the first and second color filter materials to the substrate.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/163,068, filed Jun. 27, 2008, which claims the benefit of U.S.Application No. 60/946,863, filed Jun. 28, 2007. This application isalso related to

-   -   (a) U.S. Pat. No. 7,667,684; and    -   (b) U.S. Pat. No. 7,339,715.

The entire contents of the co-pending application and patents, and ofall other U.S. patents and published and copending applicationsmentioned below, are herein incorporated by reference.

BACKGROUND OF INVENTION

This invention relates to processes for the production of electro-opticdisplays and for filters for use in such displays.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedby applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium isalso typically bistable.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. PatentPublication Nos. 2005/0259068, 2006/0087479, 2006/0087489, 2006/0087718,2006/0209008, 2006/0214906, 2006/0231401, 2006/0238488, 2006/0263927 andU.S. Pat. Nos. 7,321,459 and 7,236,291. Such gas-based electrophoreticmedia appear to be susceptible to the same types of problems due toparticle settling as liquid-based electrophoretic media, when the mediaare used in an orientation which permits such settling, for example in asign where the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspending medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545;6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333;6,704,133; 6,710,540; 6,721,083; 6,724,519; 6,727,881; 6,738,050;6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068;6,825,829; 6,825,970; 6,831,769; 6,839,158; 6,842,167; 6,842,279;6,842,657; 6,864,875; 6,865,010; 6,866,760; 6,870,661; 6,900,851;6,922,276; 6,950,200; 6,958,848; 6,967,640; 6,982,178; 6,987,603;6,995,550; 7,002,728; 7,012,600; 7,012,735; 7,023,420; 7,030,412;7,030,854; 7,034,783; 7,038,655; 7,061,663; 7,071,913; 7,075,502;7,075,703; 7,079,305; 7,106,296; 7,109,968; 7,110,163; 7,110,164;7,116,318; 7,116,466; 7,119,759; 7,119,772; 7,148,128; 7,167,155;7,170,670; 7,173,752; 7,176,880; 7,180,649; 7,190,008; 7,193,625;7,202,847; 7,202,991; 7,206,119; 7,223,672; 7,230,750; 7,230,751;7,236,790; 7,236,792; 7,242,513; 7,247,379; 7,256,766; 7,259,744;7,280,094; 7,304,634; 7,304,787; 7,312,784; 7,312,794; 7,312,916;7,237,511; 7,339,715; 7,349,148; 7,352,353; 7,365,394; and 7,365,733;and U.S. Patent Applications Publication Nos. 2002/0060321;2002/0090980; 2003/0102858; 2003/0151702; 2003/0222315; 2004/0105036;2004/0112750; 2004/0119681; 2004/0155857; 2004/0180476; 2004/0190114;2004/0257635; 2004/0263947; 2005/0000813; 2005/0007336; 2005/0012980;2005/0018273; 2005/0024353; 2005/0062714; 2005/0099672; 2005/0122284;2005/0122306; 2005/0122563; 2005/0134554; 2005/0151709; 2005/0152018;2005/0156340; 2005/0179642; 2005/0190137; 2005/0212747; 2005/0253777;2005/0280626; 2006/0007527; 2006/0038772; 2006/0139308; 2006/0139310;2006/0139311; 2006/0176267; 2006/0181492; 2006/0181504; 2006/0194619;2006/0197737; 2006/0197738; 2006/0202949; 2006/0223282; 2006/0232531;2006/0245038; 2006/0262060; 2006/0279527; 2006/0291034; 2007/0035532;2007/0035808; 2007/0052757; 2007/0057908; 2007/0069247; 2007/0085818;2007/0091417; 2007/0091418; 2007/0109219; 2007/0128352; 2007/0146310;2007/0152956; 2007/0153361; 2007/0200795; 2007/0200874; 2007/0201124;2007/0207560; 2007/0211002; 2007/0211331; 2007/0223079; 2007/0247697;2007/0285385; 2007/0286975; 2007/0286975; 2008/0013155; 2008/0013156;2008/0023332; 2008/0024429; 2008/0024482; 2008/0030832; 2008/0043318;2008/0048969; 2008/0048970; 2008/0054879; 2008/0057252; and2008/0074730; and International Applications Publication Nos. WO00/38000; WO 00/36560; WO 00/67110; and WO 01/07961; and EuropeanPatents Nos. 1,099,207 B1; and 1,145,072 B1.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, theaforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat.Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856.Dielectrophoretic displays, which are similar to electrophoreticdisplays but rely upon variations in electric field strength, canoperate in a similar mode; see U.S. Pat. No. 4,418,346. Other types ofelectro-optic displays may also be capable of operating in shutter mode.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

Other types of electro-optic media may also be useful in the presentinvention.

Many types of electro-optic media are essentially monochrome, in thesense that any given medium has two extreme optical states and a rangeof gray levels lying between the two extreme optical states. As alreadyindicated, the two extreme optical states need not be black and white.For example, one extreme optical state can be white and the other darkblue, so that the intermediate gray levels will be varying shades ofblue, or one extreme optical state can be red and the other blue, sothat the intermediate gray levels will be varying shades of purple.

There is today an increasing demand for full color displays, even forsmall, portable displays; for example, most displays on cellulartelephones are today full color. To provide a full color display usingmonochrome media, it is either necessary to place a color filter arraywhere the display can be viewed through the color filter array, or toplace areas of different electro-optic media capable of displayingdifferent colors adjacent one another.

FIG. 1 of the accompanying drawings is a schematic section through acolor electrophoretic display (generally designated 100) comprising abackplane 102 bearing a plurality of pixel electrodes 104. To thisbackplane 102 has been laminated an inverted front plane laminate asdescribed in the aforementioned 2007/0109219, this inverted front planelaminate comprising a monochrome electrophoretic medium layer 106 havingblack and white extreme optical states, an adhesive layer 108, a colorfilter array 110 having red, green and blue areas aligned with the pixelelectrodes 104, a substantially transparent conductive layer 112(typically formed from indium-tin-oxide, ITO) and a front protectivelayer 114.

In the display 100, the electrophoretic layer 106 is of course not 100percent reflective, and the saturation of the color filter elements inthe array 110 must be reduced to allow enough light to pass through thearray 110, reflect from the electrophoretic layer 106, and returnthrough the array 110. However, using a color filter array does enable asingle black/white electro-optic medium to provide a full color display,and it is typically easier to control the color gamut of a display byvarying the colors in a color filter array than by varying the colors ofelectro-optic media, there being far more materials available for use incolor filter arrays than in most electro-optic media.

Forming a color filter array such as the array 110 shown in FIG. 1 isnot easy, especially in high resolution displays having resolutions of(say) 100 lines per inch (4 lines per mm) or more, since if such arraysare formed using red, green and blue lines, the individual colored lineswill be only about 1/300 inch (about 80 μm) wide. Forming such finecolored lines using conventional printing techniques is difficult,especially since many printing techniques allow the printed material tospread laterally after printing. In a color filter array, it is highlydesirable that the colored lines touch but do not overlap, since anygaps between adjacent lines will produce in effect an unanticipatedwhite area in the color filter array and will result in a (typicallynon-uniform) decrease in color saturation, whereas any overlap willcause color distortion in the final display.

Accordingly, there is a need for improved processes for forming colorfilter arrays in which differently colored areas touch but do notoverlap, and this invention seeks to provide such improved processes.

A further difficulty in color filter arrays is aligning such arrays withthe sub-pixel electrodes of the display's backplane. In most prior artmethods for manufacturing color filter arrays, the color filter array ismanufactured as a separate integer, which may be laminated to amonochrome display, or an electro-optic medium may be coated over thecolor filter array and the resultant sub-assembly laminated to abackplane. Especially in high resolution displays, maintaining thenecessary alignment between the color filter array and the sub-pixelelectrodes is difficult, and this problem becomes especially acute inthin, flexible electro-optic displays. It would be advantageous if colorfilter arrays could be formed in alignment with sub-pixel electrodes,and preferred processes of the present invention can form such alignedcolor filter arrays.

SUMMARY OF THE INVENTION

In a first aspect, this invention provides a process for depositingfirst and second color filter materials on a substrate, the processcomprising:

depositing on the substrate a coating of a material having a surfacecharacteristic capable of being modified by radiation;

applying radiation to a first area of the coating but not to a secondarea thereof;

depositing a flowable form of the first color filter material on to thefirst area of the coating;

converting the flowable form of the first color filter material on thefirst area of the coating to a non-flowable form;

applying radiation to the second area of the coating; and

depositing the second color filter material on to the second area of thecoating.

This first process of the present invention may hereinafter forconvenience be called the “surface modification process” or “SM process”of the invention.

In a second aspect, this invention provides a process for depositingfirst and second color filter materials on a substrate, the processcomprising:

depositing the first color filter material on a first donor sheet, thefirst donor sheet absorbing radiation such that exposure of a first areaof the first donor sheet to the radiation will cause the first colorfilter material overlying the first area to become detached from thefirst donor sheet;

depositing the second color filter material on a second donor sheet, thesecond donor sheet absorbing radiation such that exposure of a secondarea of the second donor sheet to the radiation will cause the secondcolor filter material overlying the second area to become detached fromthe second donor sheet;

bringing the first donor sheet adjacent the substrate with the firstcolor filter material facing the substrate, and applying radiation tothe first area of the first donor sheet, thereby causing the first areaof the first color filter material to become detached from the firstdonor sheet and deposited on a first area of the substrate; and

bringing the second donor sheet adjacent the substrate with the secondcolor filter material facing the substrate, and applying radiation tothe second area of the second donor sheet, thereby causing the secondarea of the second color filter material to become detached from thesecond donor sheet and deposited on a second area of the substrate.

This second process of the present invention may hereinafter forconvenience be called the “donor sheet transfer process” or “DSTprocess” of the invention.

In a third aspect, this invention provides a process for depositingfirst and second color filter materials on a substrate, the processcomprising:

providing the first color filter material in a liquid form;

-   -   providing a first plate cylinder having at least one raised        portion and at least one recessed portion;

forming a layer of the liquid form of the first color filter material onthe at least one raised portion of the plate cylinder but not on the atleast one recessed portion thereof;

transferring the liquid form of the first color filter material from theplate cylinder to a first area of the substrate;

providing the second color filter material in a liquid form;

-   -   providing a second plate cylinder having at least one raised        portion and at least one recessed portion;

forming a layer of the liquid form of the second color filter materialon the at least one raised portion of the plate cylinder but not on theat least one recessed portion thereof; and

transferring the liquid form of the second color filter material fromthe plate cylinder to a second area of the substrate.

This third process of the present invention may hereinafter forconvenience be called the “flexographic process” of the invention.

The color filter materials used in the processes of the presentinvention may be any material useful in forming a color filter arraysuitable for use in an electro-optic display. Typically, the colorfilter material will be used in a form which has substantially the samecolor as the area of the color filter array derived from the colorfilter material; some minor change in color may of course occur duringconversion of a flowable or liquid form of a color filter material to asolid state. However, the color filter materials may also be used in aprecursor form which is essentially colorless but develops the necessarycolor after deposition on the substrate; for example, the color filtermaterial may contain a thermally activated dye precursor which developscolor when the color filter material is heated on the substrate toconvert it to a solid form. In the case of the donor sheet transferprocess, the precursor can be radiation-sensitive such that theradiation used to transfer the color filter material from the donorsheet to the substrate also converts the color precursor to its coloredform. The color filter materials used in the present invention willtypically have differing colors in the normal sense of that term, i.e.,different absorption profiles in the visible range, but the presentprocesses can also be used to provide “pseudo-color filter arrays” inthe sense of arrays of materials having different absorption profiles ina non-visible range. If the color filter materials used do havedifferent pseudo colors such that they are not readily distinguishableby eye, it may be convenient to provide them with different visiblecolors to facilitate inspection; the visible colors may or may not befugitive in the sense of being removable, for example by exposure toheat or radiation, prior to use of the pseudo-color display.

The processes of the present invention may be carried out at variousdifferent stages in the construction of electro-optic displays. Forexample, provided the electro-optic medium used in the display is eitherlight transmissive or operated in shutter mode, the substrate used ineach process may be a backplane bearing at least first and second setsof electrodes, and the first and second color filter materials may bedeposited aligned with the first and second sets of electrodesrespectively. An electro-optic medium, front electrode and, typically, aprotective layer can then be laminated over the color filter array toform a finished display. Alternatively and more commonly, the substrateused in each process may be a light-transmissive electrically-conductivelayer, typically supported on a light-transmissive supporting layer;after deposition of the first and second color filter materials, theresultant supporting layer/electrode layer/color filter arraysub-assembly may be coated with an electro-optic medium, and an adhesivelayer and release sheet laminated over the electro-optic medium to forma front plane laminate as described in the aforementioned U.S. Pat. No.6,982,178. In another variant of the present processes, the color filtermaterial may be deposited on a substrate comprising a release sheetwhich may or may not previously have been coated with an adhesive layer.The resultant release sheet/color filter material or releasesheet/adhesive layer/color filter material sub-assembly can then havethe exposed surface of the color filter material laminated to anelectro-optic medium to form a double release sheet as described in theaforementioned 2004/0155857. The release sheet/color filter material orrelease sheet/adhesive layer/color filter material sub-assembly couldalso be used to form an inverted from plane laminate as described in theaforementioned 2007/0109219.

This invention extends to the color filter arrays produced by theprocesses of the invention, and to electro-optic displays, front planelaminates, inverted front plane laminates and double release filmsproduced from such color filter arrays. The displays of the presentinvention may be used in any application in which prior artelectro-optic displays have been used. Thus, for example, the presentdisplays may be used in electronic book readers, portable computers,tablet computers, cellular telephones, smart cards, signs, watches,shelf labels and flash drives.

All the accompanying drawings are schematic and not to scale. Inparticular, for ease of illustration, the thicknesses of the variouslayers in the drawings do not correspond to their actual thicknesses.Also, in all the drawings, the thicknesses of the various layers aregreatly exaggerated relative to their lateral dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS

As already mentioned, FIG. 1 of the accompanying drawings is a schematicsection through a color electrophoretic display.

FIG. 2 is a schematic side elevation of a first surface modificationprocess of the present invention in which deposition of the color filtermaterial is effected by micro-pipetting.

FIGS. 3A to 3E are schematic side elevations of various stages of asecond surface modification process of the present invention.

FIGS. 4A to 4C are schematic side elevations of various stages of adonor sheet transfer process of the present invention.

FIG. 5 is a schematic elevation of a flexographic process of the presentinvention.

FIG. 6 shows a preferred display structure of the invention.

DETAILED DESCRIPTION

As already mentioned, the present invention provides three differentprocesses for depositing multiple types of color filter materials on asubstrate. These three processes will primarily be described separatelybelow, but first consideration will be given to certain issues common toall of the processes.

The processes of the invention are of course restricted to color filtermaterials which can survive the process without loss of their color orcolor-developing ability, and (in some cases) which can be prepared inthe necessary flowable or liquid forms. Similarly, a color filtermaterial which cannot be formed into a substantially solid layer, whichcan be ruptured as required to enable portions of the layer to betransferred from a donor sheet to a substrate, is not suitable for usein the donor sheet transfer process of the present invention. Displaysof the present invention may include electro-optic media of any of thetypes discussed above. For example, the electro-optic media may comprisea rotating bichromal member or electrochromic material, or anelectrophoretic material comprising a plurality of electrically chargedparticles disposed in a fluid and capable of moving through the fluidunder the influence of an electric field. The electrically chargedparticles and the fluid may be confined within a plurality of capsulesor microcells, or may be present as a plurality of discrete dropletssurrounded by a continuous phase comprising a polymeric material. Thefluid may be liquid or gaseous.

As already mentioned, the surface modification process requires flowableforms of the color filter materials, while the flexographic processrequires liquid forms and the donor sheet transfer process typicallyrequires solid forms. For obvious reasons, the final form of the colorfilter materials in each process will normally be solid. Hence, thesurface modification process and the flexographic process will normallybe carried out with an uncured form of each color filter material whichwill be cured (a term used herein to cover solvent removal,polymerization and cross-linking, as well as other known procedures forsolidifying liquids or semi-solids) to produce the color filtermaterial. Although the donor sheet transfer process uses a solid form ofeach color filter material, this form need not be identical to thatfinally present on the substrate; for example, it may be advantageous touse a partially cured form of each color filter material on the donorsheet and then to complete the curing of the color filter material onthe substrate to enhance the adhesion of the color filter material tothe substrate. In some forms of the present invention, the substrate onto which the color filter materials are originally deposited may be onlya temporary substrate (for example, the substrate could be a releasesheet from which the color filter materials are transferred to theviewing surface of a pre-formed electro-optic display) and in such casesit may be desirable to postpone final curing of the color filtermaterials until they are transferred to their final substrate.

Although the processes of the present invention have been defined aboveas requiring only two different types of color filter materials, inpractice the present processes will normally be used for creating fullcolor displays, and hence the processes will typically be used withthree or four (or even possibly more) different types of color filtermaterials. For example, the present processes can be used to createcolor filter arrays of the red/green/blue, red/green/blue/white,cyan/magenta/yellow and cyan/magenta/yellow/white types.

Electro-optic displays produced by the processes of the presentinvention can incorporate any of the optional features, such as barrierand protective layers, edge seals etc. described in the aforementioned EInk and MIT patents and applications.

Section A: Surface Modification Process

In the surface modification process of the present invention, a coatingof a material having a surface characteristic capable of being modifiedby radiation is used to control the spread of a flowable color filtermaterial across a substrate. In order to create an array of regions ofdifferent color filter materials, one must be able to pattern smallamounts of the materials very precisely. Dispensing small amounts offlowable materials can readily be accomplished; for example bymicro-pipetting, as illustrated in FIG. 2 of the accompanying drawings.If a dispensed drop 202 does not wet the substrate on to which it isdispensed, the resultant drop 204 will be confined to a small area ofthe substrate. If, on the other hand, the dispensed drop does wet thesubstrate, as indicated at drop 206, the drop may cover a large area.Neither situation is ideal for forming a precise pattern of differentcolor filter materials. The non-wetting drop 204 may fail to cover thefull area of the sub-pixel which is intended to cover (with resultantloss of color saturation) while the wetting drop 206 may spread beyondthe confines of a single sub-pixel, causing color inter-mixing. (Theterm “sub-pixel” is used herein in its conventional meaning in theimaging art to refer to the area occupied by a single color within a“pixel” which comprises a collection of at least one sub-pixel of eachcolor. For example, in an RGB display each pixel comprises threesub-pixels having red, green and blue colors, whereas in an RGBW displayeach pixel comprises four sub-pixels.) By selectively modifying thesurface energy of the substrate in accordance with the SM process of thepresent invention, the dispensed color filter material can be made towet the whole desired sub-pixel area and not adjacent sub-pixel areas.

In order to achieve the desired color filter material patterning, thesurface energy of the substrate must be selectively modified. It isimportant that the surface energy be capable of being modified with highresolution (i.e., so that the surface energy characteristics can changeover very short distances), and also the modification of surface energycharacteristics must alternate with dispensing/drying of color filtermaterial such that regions of (say) red (R), green (G), and blue (B)materials can be patterned immediately adjacent each other. Thenecessary high resolution patterning can be accomplished using lasers tomodify the surface energy characteristics; lasers can pattern at veryhigh resolutions and repeatably pattern large areas. Also, when thesubstrate used in the SM process includes a backplane, lasers canreadily be controlled by reference either to electrodes themselves or tofiducial marks on the backplane to effect the necessary alignment of thecolor filter materials with the sub-pixel electrodes. Coating materialsare known that can be turned from hydrophobic to hydrophilic by exposureto laser light, either by changing the chemistry of the coating or bydestroying a hydrophobic coating on a hydrophilic surface. In general,it is preferred for environmental reasons to use aqueous color filtermaterials, which require hydrophilic areas on which to be deposited, butobviously if solvent-based color filter materials are to be used, itwill be necessary to use a hydrophilic coating which can be converted toa hydrophobic form.

A preferred surface modification process of the present invention willnow be described with reference to FIGS. 3A to 3E of the accompanyingdrawings. As shown in FIG. 3A, the process begins with a substrate(generally designated 300) which is in the form of a complete monochromedisplay comprising a backplane 302 bearing (in order) sub-pixelelectrodes 304, a rear adhesive layer 306, a layer 308 of electro-opticmedium, a front adhesive layer 310, a light-transmissive electrode layer312, and a protective layer 314. The exposed surface (the top surface asillustrated in FIG. 3A) of protective layer 314 is coated with ahydrophobic surface treatment 316 that can be converted to a hydrophilicform by exposure to laser radiation. In the first step of the process,the coating 316 is imagewise exposed to laser radiation to convert areas318 (FIG. 3B) of the coating to the hydrophilic form. (For ease ofillustration, FIGS. 3B to 3E show the deposition of the various colorfilter materials in the form of stripes extending perpendicular to theplane of the Figures, but of course other arrangements of the colorfilter materials can be used in desired. In particular, RGBW and CMYWsub-pixels may often be arranged in 2×2 arrangements of sub-pixels toform single pixels.) Next, as illustrated in FIG. 3C, a controlledamount of a first color filter material 320 (say, a red color filtermaterial) is dispensed in liquid form on to each of the hydrophilicareas 318. Since each drop of the red material 320 wets the hydrophilicarea 318 on to which it is dispensed, the red material 320 spreadsacross the entire hydrophilic area 318, but since the remaining parts ofthe coating 316 remain hydrophobic, the red material 320 cannot spreadbeyond the edges of the hydrophilic area 318. The red material 320 isthen dried or otherwise cured to form a dried red layer 320A completelycovering each area 318.

The laser radiation is then again applied, as indicated in FIG. 3D toconvert areas 322 of the coating 316 (the areas 322 lying adjacent thecoated areas 318) to the hydrophilic form, and then a second colorfilter material 324 (say, a green color filter material—see FIG. 3D) isdispensed in liquid form on to each of the hydrophilic areas 322, anddried or otherwise cured to form a dried green layer 324A completelycovering each area 322 (FIG. 3E). Note that the green material 324 isstrictly confined to areas 322 by, on the one hand, the remaininghydrophobic areas of the coating 316 and, on the other hand, the driedred layer 320A produced in the earlier step.

Although not shown in FIGS. 3A to 3E, the last step of the process isthe use of laser radiation to convert the remaining areas of the coating316 to their hydrophilic form, and the dispensing and drying of a thirdcolor filter material (say a blue color filter material) on the areas ofthe substrate not covered by the dried red and green layers 320A and324A respectively. Note that, in this step of the process, the spreadingof the blue color filter material is controlled on both sides by thepreviously formed dried red and green layers 320A and 324A.

Several characteristics are critical to forming a color filter materialpattern with high resolution. The viscosity and uniformity of theflowable material dispensed must allow dispensing without clogging anynozzle (for example, an ink jet or micro-pipette nozzle) used for thedispensing. To create droplets of reproducible size, the surface energyof the substrate must be controlled to allow droplets to “snap off”correctly, i.e., be accurately confined to the desired areas of thesubstrate. Drying and/or curing must be complete enough that subsequentwet coatings do not disturb the patterning of previous layers; suchnon-disturbance can be enhanced by curing the color filter materiallayers between dispensing steps.

Once the desired pattern of color filter materials is complete, anadhesive can be coated or laminated over the color filter materials toallow the materials to be adhered to another component of the finaldisplay, for example a protective sheet.

As already indicated, the SM process of the present invention can beused in various ways in the manufacture of a finished electro-opticdisplay. It is presently preferred that the color filter materials becoated directly on to a monochrome display as illustrated in FIGS.3A-3E, this display typically being formed by laminating a front planelaminate to a backplane, which can be rigid or flexible. This gives thehighest display resolution, and has the advantage that any necessaryultra-violet filter layers, barriers and edge seals can be in place andinspected before the color filter array is added, thus providing a verypractical method for creating a color display from an existingmonochrome display. A thin front substrate can be used to reduceparallax between the color filter array and the electro-optic medium.Accurate alignment of the various areas of color filter material withthe sub-pixel electrodes on the backplane can be achieved by providingthe backplane with fiducial marks which can be detected and used tocontrol the application of the laser radiation, thus avoiding anyfurther alignment steps. Alternatively, as already discussed, the SMprocess can be carried out using as a substrate a light-transmissiveelectrode layer (for example, the substrate can be a front planelaminate not yet laminated to a backplane) or a release sheet. If the SMprocess is carried out on a light-transmissive electrode layer, anelectro-optic medium and a lamination adhesive layer can be laminatedover the electrode layer to form a “classic” front plane laminate, asdescribed in the aforementioned U.S. Pat. No. 6,982,178. If the SMprocess is carried out on a release sheet, an electro-optic medium maybe coated over the color filter array, or a lamination adhesive layercan be laminated over the color filter array and the release sheet/colorfilter array sub-assembly thus converted to a front plane laminate,double release sheet or inverted front plane laminate. When theresulting structure is subsequently laminated to a backplane, thelamination should of course be effected to that the areas of the variouscolor filter materials are accurately aligned with the sub-pixelelectrodes of the backplane.

The SM process can achieve very high resolution (of the order ofmicrons), which is compatible with high resolution commercial TFTbackplanes. The SM method is additive (i.e., all the color filtermaterial applied ends up in the final display, no stripping of appliedcolor filter material being required), thus making maximum use of colorfilter material. The laser patterning used in the SM process can be usedto compensate for distortions common in large plastic substrates, thusallowing high resolution patterning over such large substrates.Furthermore, laser patterning is relatively inexpensive, can accommodatea wide range of sizes of substrates, and (since the patterning issoftware controlled) allows design changes to be implemented quickly.Finally, laser patterning can be used with inexpensive, room temperatureprocessable substrates, for example poly(ethylene terephthalate).

Part B: Donor Sheet Transfer Process

The donor sheet transfer process of the present invention uses radiationto transfer selected areas of a layer of color filter material on adonor sheet to a substrate by imagewise application of radiation to thedonor sheet. Typically, the donor sheet will comprise a radiationabsorbing coating, which may expand or vaporize in any known manner todetach the color filter material from the donor sheet. A separate donorsheet is used for each color filter material to be deposited on thesubstrate. The process allows for deposition of a small area of colorfilter material in a precise location and deposition of a preciselycontrolled thickness of color filter material.

A preferred DST process of the present invention will now be describedwith reference to FIGS. 4A to 4C. As shown in FIG. 4A, the first step ofthe process is the creation of a donor sheet by applying to a carrier402 a coating 404 capable of absorbing laser radiation. For example, alaser having a wavelength of about 800 nm may be used and the coating404 optimized to absorb this wavelength. Next, as shown in FIG. 4B, auniform coating 406 of the color filter material is coated over theradiation-absorbing coating 404. The color filter material coating 406may be deposited in liquid or flowable form and subsequently dried orcured to provide a mechanically coherent layer of color filter materialon the radiation-absorbing coating 404. If desired, a thin layer ofadhesive can be coated over the radiation-absorbing coating 404 toimprove the adhesion of the color filter material coating 406 to thecoating 404. The completed donor sheet shown in FIG. 4B is now ready foruse.

Next, as shown in FIG. 4C, the donor sheet is brought adjacent asubstrate 300, with the color filter material layer 406 facing thesubstrate 300. (FIG. 4C shows the DST process being used with the samesubstrate 300 as shown in FIG. 3A, except that the coating 316 is notpresent since it is not needed in the DST process, and this substratehas already been described in detail in Part A above.) A very shortpulse (typically of the order of picoseconds) of laser radiation isapplied imagewise through the carrier 402 (which must of course besubstantially transmissive of the radiation used), and is absorbed inthe coating 404, causing this coating to expand and/or vaporize and/orchemically decompose to sever an area of the color filter material fromthe coating 406 and cause this area of color filter material to adhereto the protective layer 314 of substrate 300. (For ease of illustration,FIG. 4C shows a small gap between the donor sheet and the substrate. Inpractice, the two sheets are normally in contact with one another duringthe DST process.) The surface of the substrate 300 on which the colorfilter material is deposited may of course optionally be treated with acoating to improve the adhesion of the color filter material thereto.Alignment of the color filter material with the sub-pixel electrodes ofthe substrate may be effected by providing fiducial marks on thesubstrate and using these marks to control the laser applied to thedonor sheet, as described above.

At this point, only one color filter material has been applied to thesubstrate 300. To produce a full color display, the step of FIG. 4C isrepeated with two or more additional donor sheets to place additionalcolor filter materials on the substrate 300, thus providing a full colorelectro-optic array of sub-pixels on the substrate 300.

Once the desired pattern of color filter materials on the substrate iscomplete, an adhesive can be coated or laminated over the color filtermaterials to allow the materials to be adhered to another component ofthe final display, for example an electro-optic medium layer.

As already indicated, the DST process of the present invention can beused in various ways in the manufacture of a finished electro-opticdisplay. It is presently preferred that the color filter materials bedeposited directly on to a monochrome display, as illustrated in FIGS.4A-4C, this display typically being formed by laminating a front planelaminate to a backplane, which can be rigid or flexible. This gives thehighest display resolution, and has the advantage that any ultra-violetfilter layers, barriers and edge seals can be in place and inspectedbefore the color filter array is added, thus providing a very practicalmethod for creating a color display from an existing monochrome display.A thin front substrate can be used to reduce parallax between the colorfilter array and the electro-optic medium. Accurate alignment of thevarious areas of color filter material with the sub-pixel electrodes onthe backplane can be achieved by providing the backplane with fiducialmarks which can be detected and used to control the application of thelaser radiation, thus avoiding any further alignment steps.Alternatively, as already discussed, the DST process can be carried outusing as a substrate a light-transmissive electrode layer (for example,the substrate can be a front plane laminate not yet laminated to abackplane) or a release sheet. If the DST process is carried out on alight-transmissive electrode layer, an electro-optic medium and alamination adhesive layer can be laminated over the electrode layer toform a “classic” front plane laminate, as described in theaforementioned U.S. Pat. No. 6,982,178. If the DST process is carriedout on a release sheet, an electro-optic medium may be coated over thecolor filter array, or a lamination adhesive layer can be laminated overthe color filter array and the release sheet/color array sub-assemblythus converted to a front plane laminate, double release sheet orinverted front plane laminate. When the resulting structure issubsequently laminated to a backplane, the lamination should of coursebe effected to that the areas of the various color filter materials areaccurately aligned with the sub-pixel electrodes of the backplane.

The DST process can achieve very high resolution (of the order ofmicrons), which is compatible with high resolution commercial TFTbackplanes. The uniformity of the color filter materials layer in thefinal display is controlled by the uniformity of the layer of colorfilter material on the donor sheet, and the donor sheet can be chosen tomaximize such coating uniformity. The transfer of the color filtermaterial from the donor sheet is a “dry” process, so no subsequentdrying or curing step is required; there need be no period during whichthe deposited color filter material is tacky and could becomecontaminated by dust etc. sticking to a tacky layer, and there is nopossibility of deposition of liquid or flowable material disturbingpreviously-deposited color filter material. The radiation absorbinglayer used in the preferred DST process described above minimizes energytransfer to the color filter material and to any electro-optic mediumpresent in the substrate and thus minimizes possible radiation damage tothese materials. The laser patterning used in the DST process can beused to compensate for distortions common in large plastic substrates,thus allowing high resolution patterning over such large substrates.Furthermore, laser patterning is relatively inexpensive, can accommodatea wide range of sizes of substrates, and (since the patterning issoftware controlled) allows design changes to be implemented quickly.Finally, laser patterning can be used with inexpensive, room temperatureprocessable substrates, for example poly(ethylene terephthalate).

Part C: Flexographic Process

The flexographic process of the present invention essentially modifiesknown flexographic printing technology to apply multiple types of colorfilter material to form a color electro-optic display.

Flexographic printing is commonly used to create high quality colorprints requiring registration of multiple colored ink layers (typicallycyan, magenta, yellow, and black); the process inherently has highresolution of the order of microns to tens of microns. The basic processis shown in FIG. 5.

As shown in that Figure, in the flexographic process of the presentinvention the image to be printed (for example, an array of red colorfilter elements) is created on a patterned plate cylinder 502 havingraised and recessed areas. A fluid film 504 of the appropriate colorfilter material (for example, a mixture of a red dye and a liquidpolymer or oligomer) is picked up from a pan 506 by a fountain roll 508and transferred in a thin layer to an Anilox roll 510. The Anilox roll510 in turn transfers the thin, uniform layer of color filter materialto the plate cylinder 502 such that the liquid material 504 istransferred only to the raised areas of the plate cylinder. A web ofsubstrate 512 passes between an impression cylinder 514 and the platecylinder 502 and the color filter material 504 is transferred from theraised areas of plate cylinder 502 to the substrate 512.

A single station, as shown in FIG. 5, prints only a single colored inkor a single color filter material. The substrate 512 passes through asequence of such stations, which each apply an additional color filtermaterial of a differing color in registry with the pattern previouslyprinted on the substrate. Several commercial variations of flexographicprinting exist, including one in which the liquid to be printed isdoctor bladed on the Anilox roll 510 to achieve a more uniform coating.

Several characteristics of the liquid being printed are critical tomaking a print with high resolution. Depending on the processcharacteristics, viscosity values of 10-10,000 cP can be used, though aviscosity of the order of thousands of centipoise is commonly used.Other rheological properties (shear thickening/thinning) may also beimportant. Wetting of the substrate by the liquid being printed must becontrolled such that a sub-pixel does not bleed into an adjacentsub-pixel area. To achieve this, the surface energy of the printedmaterial and the substrate must be matched, and any necessaryadjustments can be made by adding surfactant to the printing liquid orby pre-treating the substrate to accept the liquid. Drying or curing ofthe printed liquid must be sufficiently complete that subsequentlyprinted liquids do not disturb previously printed materials; this is afunction of printing speed and imprint load. The liquid printed can bewater or solvent based, though some solvent in the mixture is preferredto increase drying speed. The printed liquid can be cured thermally orwith ultra-violet radiation to prevent subsequent printing fromdisturbing previously printed materials.

Once the desired pattern of color filter materials is complete, anadhesive can be coated or laminated over the color filter materials toallow the coated substrate to be adhered to another component of thefinal display, for example a layer of electro-optic medium.

As already indicated, the flexographic process of the present inventioncan be used in various ways in the manufacture of a finishedelectro-optic display. A preferred display structure (generally designed600) is shown in FIG. 6 of the accompanying drawings. This Figure showsa color filter array comprising red, green and blue areas 602R, 602G and602B respectively formed on a substrate comprising the same layers asthe substrate 300 as described above, except that the layer 316 isomitted. The color display 600 is formed by flexographic printing of thered, green and blue areas 602R, 602G and 602B directly on to theprotective layer 314 of substrate 300 in alignment with sub-pixelelectrodes 304R, 304G and 304B respectively. The protective layer 314 iskept thin to reduce parallax between the color filter array and theelectro-optic medium 308. The color filter materials are printeddirectly on to the substrate, which may be in the form of a continuousflexible web or (for example) in the form of a flat glass plate whichcan be translated under a plate cylinder synchronously with the rotationof the plate cylinder. Accurate alignment of the various areas of colorfilter materials with the sub-pixel electrodes can be achieved byproviding the substrate with fiducial marks which can be detected andused to control the printing process. Alternatively, as alreadydiscussed, the flexographic process can be carried out using as asubstrate a light-transmissive electrode layer or a release sheet. Ifthe flexographic process is carried out on a light-transmissiveelectrode layer, an electro-optic medium and a lamination adhesive layercan be laminated over the electrode layer to form a “classic” frontplane laminate, as described in the aforementioned U.S. Pat. No.6,982,178. If the flexographic process is carried out on a releasesheet, an electro-optic medium may be coated over the color filterarray, or a lamination adhesive layer can be laminated over the colorfilter array, and the release sheet/color filter array sub-assembly thusconverted to a front plane laminate, double release film or invertedfront plane laminate. When the resulting structure is subsequentlylaminated to a backplane, the lamination should of course be effected sothat the areas of the various color filter materials are accuratelyaligned with the sub-pixel electrodes of the backplane.

The flexographic process can achieve very high resolution (of the orderof microns), which is compatible with high resolution commercial TFTbackplanes. The flexographic method is additive (i.e., all the colorfilter material applied ends up in the final display, no stripping ofapplied color filter material being required), thus making maximum useof color filter materials. The investment cost for flexographic printingis commercially reasonable (about US$1 million for a high end four colorapparatus), and considerable smaller than the investment required forother high resolution patterning methods such as photolithography. Thislow investment cost is especially reasonable in view of the highthroughput of flexographic printing apparatus, which typically runs atabout 100-200 feet per minute (about 30-60 meters per minute) and isthus very economical for high volume production Finally, theflexographic process can be used with inexpensive, room temperatureprocessable substrates, for example poly(ethylene terephthalate).

Numerous changes and modifications can be made in the preferredembodiments of the present invention already described without departingfrom the scope of the invention. Accordingly, the foregoing descriptionis to be construed in an illustrative and not in a limitative sense.

1. A color electrophoretic display comprising: an electrophoreticmedium; a front electrode; and an opaque backplane comprising first andsecond color filter materials.
 2. The color electrophoretic display ofclaim 1, wherein the electrophoretic display is produced by laminatingthe electrophoretic medium and the front electrode to the backplane. 3.The color electrophoretic display of claim 1, wherein the opaquebackplane is produced by: providing an opaque backplane comprising firstand second sets of electrodes; depositing on the backplane a coating ofa material having a surface characteristic capable of being modified byradiation; applying radiation to a first area of the coating alignedwith the first set of electrodes but not to a second area of thecoating, said second area of the coating being aligned with the secondset of electrodes; depositing a flowable form of a first color filtermaterial on to the first area of the coating; converting the flowableform of the first color filter material on the first area of the coatingto a non-flowable form; applying radiation to the second area of thecoating; and depositing a second color filter material on to the secondarea of the coating.
 4. The color electrophoretic display of claim 1,wherein the electrophoretic medium comprises a plurality of electricallycharged particles disposed in a fluid and capable of moving through thefluid under the influence of an electric field.
 5. The colorelectrophoretic display of claim 4, wherein the electrically chargedparticles and the fluid are confined within a plurality of capsules ormicrocells.
 6. The color electrophoretic display of claim 4, wherein theelectrically charged particles and the fluid are present as a pluralityof discrete droplets surrounded by a continuous phase comprising apolymeric material.
 7. The color electrophoretic display of claim 4,wherein the fluid is gaseous.
 8. The color electrophoretic display ofclaim 1, further comprising a protective layer.
 9. An electronic bookreader, portable computer, tablet computer, cellular telephone, smartcard, sign, watch, shelf label or flash drive comprising a colorelectrophoretic display according to claim
 1. 10. A color electro-opticdisplay comprising a layer of electro-optic medium and a color filterproduced by a process comprising: depositing on a substrate a coating ofa material having a surface characteristic capable of being modified byradiation; applying radiation to a first area of the coating but not toa second area thereof; depositing a flowable form of a first colorfilter material on to the first area of the coating; converting theflowable form of the first color filter material on the first area ofthe coating to a non-flowable form; applying radiation to a second areaof the coating; and depositing a second color filter material on to thesecond area of the coating.
 11. The color electro-optic displayaccording to claim 10, wherein the electro-optic material comprises arotating bichromal member or electrochromic material.
 12. The colorelectro-optic display according to claim 10, wherein the electro-opticmaterial comprises an electrophoretic material comprising a plurality ofelectrically charged particles disposed in a fluid and capable of movingthrough the fluid under the influence of an electric field.
 13. Thecolor electro-optic display according to claim 12, wherein theelectrically charged particles and the fluid are confined within aplurality of capsules or microcells.
 14. The color electro-optic displayaccording to claim 12, wherein the electrically charged particles andthe fluid are present as a plurality of discrete droplets surrounded bya continuous phase comprising a polymeric material.
 15. The colorelectro-optic display according to claim 12, wherein the fluid isgaseous.
 16. An electronic book reader, portable computer, tabletcomputer, cellular telephone, smart card, sign, watch, shelf label orflash drive comprising a color display according to claim 10.